The cornea of an eye has five (5) different identifiable layers of tissue. Proceeding in a posterior direction from the anterior surface of the cornea, these layers are: the epithelium; Bowman's capsule (membrane); the stroma; Descemet's membrane; and the endothelium. Behind the cornea is an aqueous-containing space called the anterior chamber. Importantly, pressure from the aqueous in the anterior chamber acts on the cornea with bio-mechanical consequences. Specifically, the aqueous in the anterior chamber of the eye exerts an intraocular pressure against the cornea. This creates stresses and strains that place the cornea under tension.
Structurally, the cornea of the eye has a thickness (T) that extends between the epithelium and the endothelium. Typically, “T” is approximately five hundred microns (T=500 μm). From a bio-mechanical perspective, Bowman's capsule and the stroma are the most important layers of the cornea. Within the cornea, Bowman's capsule is a relatively thin layer (e.g. 20 to 30 μm) that is located below the epithelium, within the anterior one hundred microns of the cornea. The stroma then comprises almost all of the remaining four hundred microns in the cornea. Further, the tissue of Bowman's capsule creates a relatively strong, elastic membrane that effectively resists forces in tension. On the other hand, the stroma comprises relatively weak connective tissue.
Bio-mechanically, Bowman's capsule and the stroma are both significantly influenced by the intraocular pressure that is exerted against the cornea by aqueous in the anterior chamber. In particular, this pressure is transferred from the anterior chamber, and through the stroma, to Bowman's membrane. It is known that how these forces are transmitted through the stroma will affect the shape of the cornea. Thus, by disrupting forces between interconnective tissue in the stroma, the overall force distribution in the cornea can be altered. Consequently, this altered force distribution will then act against Bowman's capsule. In response, the shape of Bowman's capsule is changed, and due to the elasticity and strength of Bowman's capsule, this change will directly influence the shape of the cornea.
It is well known that all of the different tissues of the cornea are susceptible to LIOB. Further, it is known that different tissues will respond differently to a laser beam, and that the orientation of tissue being subjected to LIOB may also affect how the tissue reacts to LIOB. With this in mind, the stroma needs to be specifically considered.
The stroma essentially comprises many lamellae that extend substantially parallel to the anterior surface of the eye. In the stroma, the lamellae are bonded together by a glue-like tissue that is inherently weaker than the lamellae themselves. Consequently, LIOB over layers parallel to the lamellae can be performed with less energy (e.g. 0.8 μJ) than the energy required for the LIOB over cuts that are oriented perpendicular to the lamellae (e.g. 1.2 μJ). It will be appreciated by the skilled artisan, however, that these energy levels are only exemplary. If tighter focusing optics can be used, the required energy levels will be appropriately lower. In any event, depending on the desired result, it may be desirable to make only cuts in the stroma. On the other hand, for some procedures it may be more desirable to make a combination of cuts and layers.
As implied above, reshaping of the cornea by weakening tissue in the stroma can be an effective way to provide refractive corrections that will improve a vision defect. Not all vision defects, however, are caused by aberrations that are symmetrical with respect to the visual axis. Indeed, the higher order aberrations are typically asymmetrical. Accordingly, it may be necessary to weaken tissue in volumes that are offset from the visual axis. With all of this in mind, and as intended for the present invention, refractive surgery is accomplished by making incisions in the stroma centered about axes parallel to the visual axis to induce a redistribution of bio-mechanical forces that will reshape the cornea.
In light of the above, it is an object of the present invention to provide methods for correcting higher order aberrations through changes in intrastromal biomechanical stress distributions for improvement of a patient's vision. Another object of the present invention is to provide methods for correcting higher order aberrations that require minimal LIOB of stromal tissue. Still another object of the present invention is to provide methods for performing ophthalmic laser surgery that create incisions having a same pattern at selected locations about the visual axis. Yet another object of the present invention is to provide methods for correcting higher order aberrations via ophthalmic laser surgery that are relatively easy to implement and comparatively cost effective.